The present invention relates generally to a spark plug and, more specifically, to a spark plug connecting structure of high insulating capacity, including the constitution of the connecting head of a spark plug and that of a plug cap.
A conventional spark plug and a spark plug connecting structure for connecting the spark plug and a high-tension resistance cord are shown in FIG. 6.
This conventional spark plug 1 has an electrode 3 and a porcelain insulator 2 keeping the electrode 3 from grounding through the engine. The spark plug connecting structure comprises a plug terminal 5 fixed to the extremity of high-tension resistance cord 4 so as to seize the electrode 3 of the spark plug 1, and a plug cap 6 covering the porcelain insulator 2, the plug terminal 5 and the free end portion of the high-tension resistance cord 4. A mold lubricant film 7 is formed between the inner surface of the plug cap 6 and the outer surface of the porcelain insulator 2 to prevent the plug cap 6 from sticking to the porcelain insulator 2 under the influence of heat.
A high voltage is applied to the plug terminal 5 and the electrode 3 of the spark plug 1 through the high-tension resistance cord 4 connected directly to or through a distributor to an ignition coil. The major part of the electrode is insulated by the porcelain insulator 2 and the plug cap 6. The recent development of internal-combustion engines has a general tendency to raise the compression ratio and to provide the internal-combustion engine with a turbosupercharger requiring increased voltage to be applied to the spark plug. Under such circumstances, the conventional spark plug and the spark plug connecting structure are not satisfactory in the sealing characteristics over the sealing length s of the porcelain insulator 2 and the plug cap 6. Furthermore, the mold lubricant film 7 formed between the plug cap 6 and the porcelain insulator 2 to prevent the plug cap 6 from sticking to the porcelain insulator 2 due to heat is liable to deteriorate the sealing effect of the plug cap 6.